How the size of adult neuron populations is regulated is not understood although the information is relevant to explaining how differences and/or similarities in neuron population sizes are effected through development. One possibility is that neuron populations are matched to their postsynaptic targets through competition for connections, and that neuronal cell death eliminates those neurons that are unsuccessful. For anuran amphibians in particular, it is unclear that this is true for the neuron populations innervating the limbs since limb size and motoneuron population size (i.e., neuron number) appear to be poorly correlated in cases where surgery altered normal development. The proposed research will examine the regulation of motoneuron numbers in the lumbar lateral motor column in three groups of the anuran amphibian, Xenopus laevis: normally-developing, propylthiouracil-treated and triploids. Two likely motoneuron number regulators (muscle fiber number and hindlimb muscle volume) and the size of the motoneuron population (cell number and mean cell size) will be measured in individuals from six developmental stages. The correlations between motoneuron population size and these two aspects of the limb have never been determined, and the variations in cell size and cell number that apparently occur between these three types of X. laevis can be used to test the idea that a size match explains motoneuron number regulation. These experiments avoid particular problems associated with surgery. If these aspects of the hindlimb do regulate neuron population size, then neuron populations of abnormal size can readily be related to developmental events that change these features of limbs. If these aspects of hindlimb size are not regulating motoneuron numbers, then other aspects of the hindlimb need to be identified and their roles tested, or other modes of neuron number regulation need to be investigated. Identifying any developmental regulators of neuron numbers will allow a better understanding of the significance that cell death has in producing appropriately size neuron populations throughout the nervous system.